This invention relates to means for cooling a high heat flux device or element such as a semiconductor device, and more particularly to a liquid-cooled electronic device which effectively eliminates heat generated from a semiconductor device used, for example, in a high-speed computer.
Recently, a high-speed design has been increasingly required for computers, and there have been developed methods of mounting a large-scale semiconductor integrations at a high density. Therefore, an amount of heat generated from semiconductor devices has become enormous, and it has become more and more important to efficiently cool the semiconductor devices. Under the circumstances, a conventional cooling device for achieving a high cooling ability is proposed in Japanese Utility Model Unexamined Publication No. 3-61350, in which semiconductor devices generating intense heat are cooled by a forced convection/boiling heat transfer which is a combination of heat transfer by force convection and heat transfer with boiling. As shown in FIG. 15, a cooling medium is ejected onto a back surface (cooling surface) of each of a number of semiconductor devices 3 (which are mounted on a substrate 1) from a cooling medium ejection port 6 provided at a distal end of a respective one of cooling medium supply members 5 communicated with a cooling medium supply header 4.
Japanese Patent Unexamined Publication No. 2-197155 discloses a structure in which holes are formed on a cold plate, and a jet of cooling medium is supplied into each hole by a tapered nozzle.
In the prior technique disclosed in the above Japanese Utility Model Unexamined Publication No. 3-61350, the low-boiling cooling medium having a low-boiling point is ejected from the cooling medium ejection port to the semiconductor device to cool the same through boiling. In this cooling structure, however, when power is put into the semiconductor device is turned on, the temperature of the semiconductor device does not rise ideally (an ideal temperature rise is shown in FIG. 14A), but overshoots transiently as shown in FIG. 14B, which is undesirable.
More specifically, since the pressure of the cooling medium at a jet-impinging area of the semiconductor device (which is generally twice larger in diameter than the ejection port) is higher than at end portions of the semiconductor device, the boiling which develops immediately after the power is turned on does not rapidly spread to the cooling medium-impinging area, so that the overall temperature of the semiconductor device is raised. Therefore, at this time, the temperature of the semiconductor device once rises abruptly. Because of this temperature rise, the cooling medium reaches the boiling point, and the boiling starts also at the impinging area. As a result, bubbles are produced from the entire surface of the semiconductor device to produce a fine convection to abruptly increase a heat transfer rate. Then, finally, a thermal equilibrium is established, so that a steady operating temperature is achieved and maintained.
The transient temperature change at this time causes the semiconductor device to repeatedly undergo a thermal stress, thereby imparting a mechanical damage to the semiconductor device to lower its reliability. Where the temperature change is large, the semiconductor device may be broken. Furthermore, in each semiconductor device, the heat transfer rate is lower at its jet-impinging area than at the other areas, and therefore the temperature becomes higher at this impinging area, so that the temperature does not become uniform over the entire area of the semiconductor device, and a temperature gradient developing at the impinging area adversely affects electrical properties. This has been the greatest obstacle to computers requiring a high-speed operation.
As described above, in the above prior arts, no consideration has been given to the reduction of the transient temperature change developing in the semiconductor device generating intense heat, and therefore such prior arts have not been satisfactory in the cooling ability.